There are two types of electroprocessing systems for electrically processing metals: namely, an electrolytic processing apparatus and an electric discharge processing machine. The first-mentioned apparatus (electrolytic processing apparatus) employs an electrolyte such as sodium nitrate and sodium chloride which fills the gap between a work and an electrode. In operation, the electrolyte is circulated at a high speed and D.C. current is made to flow from the work to the electrode while a suitable measure is taken for removing electrolytic products which would impair the stability of the electrolysis, for example intermetallic compounds, metal ions and hydrogen gas. This type of electrolytic processing apparatus is shown, for example, in Japanese Patent Laid-Open No. 71921/1986 and Japanese Patent Laid-Open No. 44228/1985.
The second-mentioned type of apparatus (electric discharge processing apparatus) employs a bath of a processing liquid such as water, kerosene or the like in which a work is disposed to oppose an electrode with a small gap therebetween. In operation, the work and the electrode are connected to each other so as to cause a momentary sparking discharge or a transient arc discharge between the work and the electrode, thereby processing the work by the energy of the discharge. This type of processing apparatus is shown, for example, in Japanese Patent Publication No. 26646/1985 and Japanese Patent Laid-Open No. 177819/1985.
The known electrolytic processing arrangements in general suffer from the following vital defect. Namely, it is impossible to obtain a uniform flow velocity of the electrolyte through the gap between the electrode and the work when the work has a complicated shape such as a three-dimensional recess with a bottom. In addition, different levels of concentration of the electrolytic products are developed between the inlet and outlet side, even when a large pressure of the electrolyte is applied to the discharge gap. This means that different portions of the discharge gap produce different processing conditions, even if the discharge current is developed uniformly over the whole area of the gap. In consequence, it becomes difficult to precisely transfer the electrode to the work and, hence, to obtain a high precision of the processed work surface.
On the other hand, the electric discharge processing arrangements in general exhibit a comparatively high level of efficiency in the range of fineness of the work surface in terms of roughness (Rmax) of up to 20 .mu.m. For attaining a higher degree of surface fineness, it is necessary to employ a very small processing current of less than IA. In consequence, the processing time is impractically long, particularly when the work has a large surface to be processed. When the area of the processed surface is large, the electrostatic capacitance between the work surface and the electrode is increased, so that it becomes difficult to delicately control the discharge current, with the result that the desired level of surface fineness is not achieved.
In order to eliminate these problems of the known apparatus, the assignee of the present application has proposed, in Japanese Patent Application No. 27616/1987, an electrolytic finish processing in which the power supply system provides pulses of a low current density in the beginning portion of the finish processing and pulses of a high current density in the later portion of the finish processing. In some cases, pulses of a high current density are intermittently applied to the work surface in order to remove any film. More specifically, a plurality of discharge switches connected between a plurality of capacitors and the work are simultaneously turned on so that electric discharges take place from all the capacitors to develop a predetermined current density of pulses between the electrode and the work.
In this type of electric power supply system, all the capacitors which have large capacitances are made to discharge by the simultaneous closing of all the discharge switches, regardless of whether the pulses are supplied at the low current density or at the high current density. In this system, therefore, a long switching time is required for charging up the capacitors when switching the current density from a low to a high density. In consequence, the processing time is impractically prolonged, and a smooth switching of the pulses is often not achieved.
Accordingly, an object of the present invention is to provide a power supply system for an electrolytic processing apparatus which is capable of efficiently conducting electric discharges from a plurality of capacitors to a work in accordance with the current density of processing pulses to be supplied, so as to finish a complicated surface on the work such as a three-dimensional shape in a sort time and with a high degree of precision, thereby producing a mirror-finished lustre surface.